This invention relates to electrically conductive pastes (herein referred to simply as "conductive pastes") which can be useful as a conductive material for the production of multi-layered ceramic substrates, as well as conductive materials and multilayered ceramic substrates themselves produced by using such a paste.
Ceramic substrates are being used commonly for mounting many kinds of electronic components to form an electronic circuit in order to produce compact electronic devices and instruments. In order to further increase the mounting density, it is becoming a common practice to stack a plurality of ceramic green sheets with circuit patterns formed on their surfaces by using a paste containing a conductive substance and to form an integral multi-layered structure by a firing process. In order to make electrical connections between such ceramic sheets, viaholes are initially formed through the ceramic green sheets by using a drill or a puncher and are filled with a conductive paste. Circuits are formed on the surfaces of the green sheets, for example, by a screen-printing process by using such a conductive paste. After the green sheets are thus prepared, they are stacked one on top of another, compressed, cut to an appropriate size and then subjected to a firing process. The conductive paste, both inside the viaholes and on the green sheets, is sintered at the same time such that the circuits inside the multi-layered ceramic substrate become connected.
As electrically conductive particles which are mixed in to form such pastes, use is frequently made of Cu because it has low specific resistance, because migration does not occur easily and also because it is inexpensive. Particles of Cu or an alloy of Cu are mixed and dispersed within an organic vehicle such as ethyl cellulose resin. As an example of paste with a Cu alloy, Japanese Patent Publication Tokkai 4-28109 has disclosed a paste comprising particles of an Ag/Cu/M alloy (where M is one or more selected from Pb, Bi and Sn) and an organic vehicle, having improved properties in electric conductivity, resistance against oxidation, solderability and resistance against migration. Japanese Patent Publication Tokkai 6-97667 has disclosed another paste for multi-layered substrates formed by a low-temperature firing process, comprising particles of an Ag/Cu alloy with increased Ag density on the surface, glass frit and an organic vehicle, having improved properties in electric conductivity, adhesion to green sheets, resistance against oxidation and resistance against migration.
In summary, prior art multi-layered ceramic substrates are produced by subjecting ceramic green sheets and a Cu paste or a Cu alloy paste to a firing process simultaneously. When subjecting a Cu paste to a firing process, however, the temperature must be kept below the melting point of Cu, or at about 1000.degree. C. As a result, the conductive particles between the multilayered ceramics and inside the viaholes cannot easily be sintered or become a densely packed structure. This tends to leave gaps (or "pores") inside the conductor, causing poor conductivity and adversely affecting the reliability of the substrate.
A similar result is obtained also when a paste with a Cu alloy is used. In the case of a Cu alloy paste also containing particles of a metal having a low melting point such as Ag, the temperature of the firing process must be kept even lower since the melting point of such an alloy is lower than that of Cu. In other words, the sintering of the alloy cannot be carried out effectively by the firing process.